Add snapshot tests for iOS 15 and audit results

[Sherlouk]

This'll include changes to the GitHub Actions workflow. We'll want to double check that everything still works.

We may also want to explore the new DocC feature set to ensure we have good coverage across the board, and offer up some basic guides on how to use the package.

[NickEntin]

Have you tested it out yet to see if anything breaks?

Xcode 13 is only available on the macOS 11 image for GitHub Actions, which is currently in private preview and I'm not sure if our org has access yet. We can definitely try it though.

We'll want to maintain Xcode 12 support for a while, so we should add an Xcode 13 build in additional to the existing jobs.

[NickEntin]

I haven't looked very deep into DocC yet, but agreed it's definitely worth exploring.

[NickEntin]

macOS 11 is now generally available, so we're unblocked on the CI side to add an iOS 15 / Xcode 13 build at this point.

@Sherlouk Did you get a chance to test it out with Xcode 13 and see if everything works alright?

[Sherlouk]

I have not yet taken a look at Xcode 13 and this library, I'm a bit behind on my own projects currently.

[NickEntin]

Now that CI is unblocked, we should add the iOS 15 build. We need to:

• Add iOS 15 to our build script and record new reference images
• Update the CI config to include an iOS 15 build
• Audit the reference images against VoiceOver on a device running iOS 15 and file issues for any changes/regressions where the behavior doesn't match